1. Field of the Invention
This invention relates to a noise reducing microphone apparatus and, in particular, to such an apparatus for reducing noise components in microphone outputs.
2. Description of the Prior Art
Most of microphones are configured to convert changes in sound pressure of an acoustic wave to mechanical vibration of a diaphragm and to activate an electro-acoustic transducer system on the basis of the vibration. Therefore, if a factor affects the diaphragm when sound is picked up by the microphone, a noise is produced.
If the factor is wind, a noise by wind (hereafter referred to as a wind noise) is produced, and if the factor is vibration, a noise by vibration (hereafter referred to as a vibration noise) is produced.
There are, for example, the following existing techniques for reducing a wind noise:
(1) the use of a windscreen PA1 (2) the use of an electro-acoustic high pass filter PA1 (3) the use of an arrangement exploiting a non-directional property in low sound ranges PA1 (1) the use of a vibration isolating mechanism PA1 (2) the use of a non-directional microphone element PA1 (3) an analog noise-canceling method PA1 (1) In the case where a windscreen is used, in general, as the outer dimension of the windscreen increases and as the distance between the microphone and the inner wall of the windscreen increases, a wind noise decreases. However, the size of the microphone apparatus increases. PA1 (2) Since a wind noise mainly consists of low band components, it is certainly effective for reducing the wind noise by using a high pass filter. However, since low band components of the sound itself are also cut in addition to the wind noise, the sound pickup quality is decreased. PA1 (3) With a non-directional microphone, in comparison with a directional microphone, the level of a wind noise decreases more. Practically, however, because of the effect of a casing surrounding the microphone, the noise is not decreased to a sufficiently low level by employing an "arrangement exploiting a non-directional property in low sound ranges". PA1 a pair of microphone units disposed in proximate locations; and PA1 subtracting means for performing subtractions of outputs from the pair of microphone units, PA1 wherein an output from one of the microphone units is supplied as the primary input signal of the adaptive noise canceller and a differential output from the pair of microphone units is supplied as the reference input signal of the adaptive noise canceller.
There are, for example, the following existing techniques for reducing a vibration noise:
The above-mentioned existing techniques for reducing a wind noise involve the following problems:
Therefore, under the present circumstances where both smaller dimension of a microphone and higher sound pickup quality of the microphone are desired, more reduction of a wind noise is difficult with only the existing techniques. This also applies to a vibration noise.
On the other hand, as a technique for eliminating a noise incorporated into a signal, adaptive noise cancelling is known (B. Widrow et al. "Adaptive noise cancelling: principles and applications" Proc. IEEE, vol. 63, no. 12, pp. 1692-1716, Dec. 1975.).
According to the technique, it is necessary to supply noise components which are strongly correlated with a noise to be eliminated as a reference input signal. However, it is very difficult in a small apparatus to supply only noises such as a wind noise as a reference input which is received from the same direction as necessary sounds.